1. Field of the Invention
The present invention relates to a head used for thermally-assisted magnetic recording in which a light propagating through a waveguide is converted into near-field light (NF-light), a magnetic recording medium is irradiated with the NF-light, thereby anisotropic magnetic field of the medium is lowered, thus data can be written. Further, the present invention relates to a magnetic recording apparatus provided with the head.
2. Description of the Related Art
As the recording densities of magnetic recording apparatuses become higher, as represented by magnetic disk apparatuses, further improvement has been required in the performance of thin-film magnetic heads and magnetic recording media. To improve the recording densities, it is necessary to decrease the size of magnetic grains that constitute a magnetic recording layer of the medium and to reduce irregularity in the boundary of record bit, thus to form minute record bits reliably. However, the decrease in size of the magnetic grains raises a problem of degradation in thermal stability of the magnetization due to the decrease in volume. As a measure against the thermal stability problem, it may be possible to increase magnetic anisotropy energy KU of the magnetic grains. However, the increase in energy KU causes the increase in anisotropic magnetic field (coercive force) of the magnetic recording medium. As a result, the head cannot write data to the magnetic recording medium when the anisotropic magnetic field (coercive force) of the medium exceeds the write field limit.
Recently, as a method for solving the problem of thermal stability, so-called a thermally-assisted magnetic recording technique is proposed. In the technique, a magnetic recording medium formed of a magnetic material with a large magnetic anisotropy energy KU is used so as to stabilize the magnetization; anisotropic magnetic field of the medium is reduced by applying heat to a portion of the medium where data is to be written; just after that, writing is performed by applying write magnetic field (write field) to the heated portion.
In the thermally-assisted magnetic recording, a technique is well known, which utilizes a near-field optical device (NF-optical device) as a metal piece that generates NF-light from plasmon excited by irradiated laser light, so-called plasmon antenna. For example, U.S. Pat. Nos. 6,768,556 and 6,649,894 disclose a technique in which NF-light is generated by irradiating a metal scatterer with light and by matching the frequency of the light with the resonant frequency of plasmon excited in the metal.
As described above, various kinds of thermally-assisted magnetic recording systems with NF-optical devices have been proposed. Meanwhile, the present inventors have devised a NF-optical device in which laser light is coupled with the NF-optical device in a surface plasmon mode to cause excited surface plasmon to propagate to the opposed-to-medium surface, thereby providing NF-light, instead of directly applying laser light to a NF-optical device. The NF-optical device is hereinafter referred to as a surface plasmon generator. In the surface plasmon generator, its temperature does not excessively rise because laser light is not directly applied to the surface plasmon generator. As a result, there can be avoided a situation in which the end of a read head element, which reaches the opposed-to-medium surface, becomes relatively far apart from the magnetic recording medium due to the thermal expansion of the NF-optical device, which makes it difficult to properly read servo signals. In addition, there can also be avoided a situation in which the light use efficiency is degraded because thermal fluctuation of free electrons increases in the NF light generator. Here, the light use efficiency is a ratio of the intensity of NF-light emitted from the NF-optical device relative to the intensity of laser light incident to the waveguide.
In a near-field light generating (NFL-generating) optical system that includes the surface plasmon generator devised by the present inventors and a waveguide through which laser light propagates, it is important to improve the light use efficiency descried above. Here, the surface plasmon generator includes a portion where the generator is coupled with the waveguide light in a surface plasmon mode. Specifically, a surface plasmon mode is induced in the portion by coupling between evanescent light that is equivalent to waveguide light seeping from the waveguide and fluctuations in charge excited on the surface of the surface plasmon generator, thereby exciting surface plasmon. Accordingly, in order to increase the light use efficiency of the NFL-generating optical system described above, it is important to enable the evanescent light to well reach a desired location in the surface plasmon generator. To that end, some artifices are needed not only in the structure of the surface plasmon generator but also in the configuration of the NFL-generating optical system including the waveguide.